FIG. 10 shows a structure of a magneto-optical recording apparatus in an invention of our earlier application.
In the drawing, 1 is a magneto-optical recording medium, 2 is a semiconductor laser, 3 is a beam splitter, 4 is a reproducing apparatus, and 5 is an external magnetic field generating device. The structure of the magneto-optical recording medium in the invention of our earlier application is also shown in FIG. 11.
In the magneto-optical recording medium, as shown in FIG. 11, the following, for example, are formed on a substrate by a method such as sputtering:
______________________________________ Dielectric layer: SiN.sub.x 650 angstroms First magnetic layer: Tb.sub.22 Fe.sub.69 Co.sub.9 800 angstroms Second magnetic layer: Gd.sub.8 Dy.sub.17 Fe.sub.60 Co.sub.15 1500 angstroms Third magnetic layer: Tb.sub.16 Fe.sub.84 200 angstroms Fourth magnetic layer: Tb.sub.30 Co.sub.70 400 angstroms Protective layer: SiN.sub.x 700 angstroms ______________________________________
Adjacent magnetic layers are here coupled by exchange forces. The first magnetic layer is the recording layer for recording and storing information. The second magnetic layer, third magnetic layer, and fourth magnetic layer do not function as information media but are additional layers that enable direct overwriting by optical modulation. The fourth magnetic layer is an initializing layer, the sublattice magnetization of which does not reverse under heating due to laser illumination within the operating range and acts in opposition to the bias magnetic field. The third magnetic layer is a control layer for shielding the exchange force from the fourth magnetic layer at high temperatures.
Next the operation will be explained.
[0] Initialization process
After formation of the layers of the magneto-optical recording medium, out of the 2.sup.4 sublattice magnetic moment configurations, the recording medium is put first and once only into the two magnetization states shown in FIG. 12 in which the magnetic moments of the transition-metal (TM) sublattices of the second magnetic layer, the third magnetic layer, and the fourth magnetic layer point down. After recording, these two states will be the recorded states. Outlined arrows in the drawings will hereinafter indicate magnetization, arrows appearing therewithin or alone will indicate the magnetic moment of the transition-metal sublattice, dotted line will indicate the state in which a magnetic wall exists between magnetic layers, and a horizontal line will indicate the state of heating above the Curie temperature, in which ferromagnetization is lost.
The bias magnetic field Hb is generated so that in the vicinity of the Curie temperature, the sublattice magnetization direction of the second magnetic layer opposes the magnetization orientation of the fourth magnetic layer. If the fourth magnetic layer is oriented in the down direction, accordingly, then the orientation will be upward if it comprises a magnetic material having a compensation temperature below the Curie temperature, and downward if it comprises a magnetic material not having a compensation temperature below the Curie temperature. In this embodiment it is generated in the upward direction.
If not explicitly stated otherwise, the word magnetization will hereinafter denote the magnetization of the transition-metal sublattice.
[1] Low-temperature operation
When the laser output is raised higher than in reproduction and the magnetic layers in the focused spot are heated to the vicinity of the Curie temperature of the first magnetic layer, as shown in FIG. 13, the magnetization direction of the second magnetic layer remains unchanged, and the magnetization of the second magnetic layer is transferred to the first magnetic layer, so the first magnetic layer becomes magnetized in the down direction.
The third magnetic layer and the fourth magnetic layer play no particular role in this process, and even if the magnetization of the third magnetic layer is lost, it is magnetized in the same direction again by the exchange coupling with the fourth magnetic layer, giving the initial "0" state.
[2] High-temperature operation
When the temperature is raised to the vicinity of the Curie temperature of the second magnetic layer as shown in FIG. 14, the first magnetic layer and third magnetic layer are demagnetized, but the magnetization direction of the fourth magnetic layer does not change. The second magnetic layer receives no exchange forces from the first magnetic layer and the third magnetic layer, but is magnetized by the external magnetic field in the up direction.
When the temperature falls below the Curie temperature of the first magnetic layer, the magnetization of the second magnetic layer is transferred to the first magnetic layer and the first magnetic layer becomes magnetized in the up direction.
When the temperature falls below the Curie temperature of the third magnetic layer, the magnetization of the third magnetic layer aligns with the fourth magnetic layer in the down direction, then the temperature falls further, and the magnetization of the second magnetic layer aligns with the magnetization of the fourth magnetic layer via the third magnetic layer, in the down direction, giving the initial "1" state.
In this way, optically modulated direct overwriting is possible by modulating only the laser light intensity.
Although prior-art magneto-optical recording media structured as above enable direct overwriting, the external magnetic field generating device is an obstacle to the further reduction of the size of the magneto-optical recording apparatus.
Another problem is that the external magnetic field generating device is disposed on the opposite side of the magneto-optical recording medium from the optical head. If double-sided media are used, it is not possible to place the optical head on the opposite side of the magneto-optical recording medium.